Blood Test Detects Down Syndrome (and More) with No Risk to Fetus

A newly developed blood DNA test given to expectant mothers can determine if an unborn baby has Down syndrome or other similar birth defects. Unlike invasive procedures such as the amniocentesis and chorionic villus sampling (CVS), the test poses no risk to the fetus.Down syndrome occurs when a child has three, rather than two, copies of the 21st chromosome. The condition causes mild to moderate impairment of cognitive ability. Common physical traits of Down syndrome are low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm. Down syndrome increases the risk for certain medical conditions such as congenital heart defects, respiratory and hearing problems, Alzheimer's disease, childhood leukemia, and thyroid conditions. One in every 733 babies is born with the condition and there are over 400,000 people living with Down syndrome in the United States.Stanford University bioengineer Stephen Quake, Ph.D., of Stanford University in California led the research team in creating a way to examine small amounts of fetal DNA that circulate in the mother's blood to look for extra chromosomes that trigger Down's syndrome and other similar birth defects. “We sequence millions of molecules from each blood sample and map the DNA back to the chromosome. And then we look to see if any chromosomes are over-represented,” Quake said.The test requires only a small blood sample from the mother, making it much safer than sticking a needle in the uterus as required by the amniocentesis, or using of CVS, which requires snipping off a small piece of the placenta for analysis. Both invasive procedures carry a risk of causing a miscarriage or birth defects.The blood test can also be done very early in a pregnancy, as early as five weeks after conception, with results available within a couple of days. With invasive procedures, which are typically done after the 15th week of pregnancy, results can take two to three weeks.Although Down syndrome is the most commonly occurring chromosomal condition, other dangerous chromosomal conditions exist that can also be detected by the new test. One such condition is Edward syndrome, a condition that is caused by the presence of three, as opposed to two, copies of chromosome 18 in a fetus or infant's cells. The syndrome occurs in about one in 3,000 pregnancies and approximately one in 6,000 live births and has a very low rate of survival due to heart abnormalities, kidney malformations, and other internal organ disorders.Another condition indicated by the new test is Patau syndrome, in which a patient has an additional chromosome 13. The condition causes numerous physical and mental abnormalities, with the most predominant being heart defects, and kills more than 80 percent of children in infancy. Patau syndrome affects approximately 1 in 12,000 live births.According to the researchers, the accuracy of the new test was demonstrated in a small study of 18 women and a single male donor where the nine women having a Down syndrome pregnancy were correctly identified. In addition, the test identified two of the women as carrying fetuses with Edward syndrome and one carrying a fetus with Patau syndrome. The test detected every genetic defect and was able to identify every normal pregnancy as well as the male donor.According to Quake in a telephone interview, “It's the first universal, noninvasive test for Down syndrome. So this should be the first step in putting an end to invasive testing procedures like amniocentesis and chorionic villus sampling.”Although there are some other noninvasive prenatal tests available, such as the ultrasound, and blood tests like the alpha-fetoprotein test that can find potential signs of a chromosomal disorder such as Down syndrome, they cannot diagnose with certainty. Quake said that these tests are “indirect and weak predictors of what's going on.”Quake said the next step is to perform a much larger study and that the new test could be widely available in two or three years, possibly becoming a routine prenatal test of a baby's health. Currently, the test would cost about $700. But that cost would drop considerably if it were widely used. The study can be found in the journal Proceedings of the National Academy of Sciences